1. Field of the Invention
The present invention relates to mixers, and more particularly relates to self-matched fundamental and subharmonic active double-balanced mi
2. Description of Related Art
Recently there has been a great amount of activity in realizing low-cost low-power integrated wireless receivers. Wireless receivers general more downconversion stages, each of which contributes to power consumption. Low-power receivers can be realized if the number of downconversion stages is reduced to one. Low-cost receivers can be realized using an inexpensive semiconductor processing technology, which typically does not feature high quality factor (high-Q) inductors. Downconversion stages use a mixer for the frequency translation from the radio frequency (RF) to the intermediate frequency (IF). If the IF frequency is not equal to 0, the receiver is generally known as superheterodyne, whereas if the frequency is equal to 0, the receiver is known as homodyne or Direct Conversion Receiver (DCR).
A conventional double-balanced Gilbert mixer, as shown in FIG. 1, is used in a downconversion stage for both superheterodyne receivers and DCRs. The use of a double-balanced Gilbert mixer in a DCR may lead to self-mixing, local oscillator (LO) leakage and DC offsets, which occur due to the same frequency of the RF and LO signals. A double-balanced Gilbert mixer can be modified to effectively address the problems associated with DCRs. A conventional active subharmonic double-balanced mixer, as shown in FIG. 2, is a modification of the double-balanced Gilbert mixer of FIG. 1. An active subharmonic double-balanced mixer alleviates the problem of self-mixing and DC offsets by using an LO signal that has a frequency that is approximately one-half the RF signal frequency.
The input of both a double-balanced Gilbert mixer and an active subharmonic double-balanced mixer needs to be matched to the nominal impedance. In order to do this, conventional mixer input matching techniques require inductive components (e.g. high-Q integrated chip inductors or off-chip inductors) to resonate the capacitive input of the mixer. However, integrated high-Q inductors are not available in inexpensive processes. In addition, they occupy large areas of the semiconductor real estate. Finally, off-chip inductors add to the assembly and packaging cost.
The applicability of a double-balanced Gilbert mixer in a DCR is limited because of the imperfect isolation between the LO and RF ports. As the LO signal is often considerably stronger than the RF signal, it leaks into the RF port through a parasitic path between the (LO and RF) ports. A LO signal coupled to the RF port mixes with the LO signal from the LO port and produces a static DC offset. A LO signal coupled to the RF port may also radiate through the antenna, due to the imperfect isolation of the receive RF path, reflect off moving objects, and penetrate the receiver. The received LO signal mixes with the LO signal from the LO port and produces a time-variable DC offset. In order to reduce DC offsets, it is advantageous to improve the isolation between the RF and LO ports.